U.S. patent number 4,405,586 [Application Number 06/321,063] was granted by the patent office on 1983-09-20 for n-secondary butyl glycine promoted acid gas scrubbing process.
This patent grant is currently assigned to Exxon Research and Engineering Co.. Invention is credited to Guido Sartori, Warren A. Thaler.
United States Patent |
4,405,586 |
Sartori , et al. |
September 20, 1983 |
N-Secondary butyl glycine promoted acid gas scrubbing process
Abstract
The present invention relates to an alkaline salt promoter
system which includes N-secondary butyl glycine and its use in acid
gas scrubbing processes.
Inventors: |
Sartori; Guido (Linden, NJ),
Thaler; Warren A. (Aberdeen, NJ) |
Assignee: |
Exxon Research and Engineering
Co. (Florham Park, NJ)
|
Family
ID: |
23249026 |
Appl.
No.: |
06/321,063 |
Filed: |
November 13, 1981 |
Current U.S.
Class: |
423/233;
174/DIG.22; 252/189; 252/190; 252/192; 423/228; 423/229; 423/232;
423/234 |
Current CPC
Class: |
B01D
53/1493 (20130101); Y10S 174/22 (20130101) |
Current International
Class: |
B01D
53/14 (20060101); B01D 053/34 (); C09K
003/00 () |
Field of
Search: |
;423/223,226,228,229,232,234 ;252/189,190,192 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4094957 |
June 1978 |
Sartori et al. |
4112050 |
September 1978 |
Sartori et al. |
4180548 |
December 1979 |
Say et al. |
4183903 |
January 1980 |
Melchior et al. |
|
Foreign Patent Documents
|
|
|
|
|
|
|
767105 |
|
Nov 1971 |
|
BE |
|
1305718 |
|
Feb 1973 |
|
GB |
|
Primary Examiner: Thomas; Earl C.
Attorney, Agent or Firm: Halluin; Albert P. Hasak; Janet
Claims
What is claimed is:
1. A process for the removal of CO.sub.2 from a gaseous stream
containing CO.sub.2 which comprises contacting said gaseous stream
(1) in an absorption step with an aqueous absorbing solution
comprising (a) a basic alkali metal salt or hydroxide selected from
the group consisting of alkali metal bicarbonates, carbonates,
hydroxides, borates, phosphates, and their mixtures, and (b) an
activator or promoter system for said basic alkali metal salt or
hydroxide comprising N-secondary butyl glycine; and (2) in a
desorption and regeneration step, desorbing at least a portion of
the absorbed CO.sub.2 from said absorbing solution.
2. The process of claim 1 wherein the basic alkali metal salt or
hydroxide is potassium carbonate.
3. The process of claim 1 wherein the aqueous solution contains 10
to about 40% by weight of said basic alkali metal salt or
hydroxide.
4. The process of claim 1 wherein the aqueous solution contains 2
to about 20% by weight of said N-secondary butyl glycine.
5. The process of claim 4 wherein the aqueous solution contains 5
to about 15% by weight of said N-secondary butyl glycine.
6. The process of claim 4 wherein the absorbing solution from the
regenerating step is recycled for reuse in the absorption step.
7. The process of claim 1, 2, 3, 4, 5 or 6 wherein the temperature
of the absorbing solution during the absoption step is in the range
from about 25.degree. to about 200.degree. C., the pressure in the
absorber ranges from about 5 to about 200 psia and the partial
pressure of the acid gas components in the feed stream ranges from
about 1.0 to about 500 psia, and wherein the temperature of the
absorbing solution during the regeneration step ranges from about
25.degree. to about 200.degree. C., and at pressures ranging from
about 16 to about 100 psia.
8. The process of claim 1, 2, 3, 4, 5, or 6 wherein the absorbing
solution additionally includes additives selected from the group
consisting of antifoaming agents, antioxidants and corrosion
inhibitors.
9. A process for the removal of CO.sub.2 from a gaseous stream
containing CO.sub.2 which comprises, in sequential steps, (1)
contacting the gaseous stream with an absorbing solution comprising
(a) from about 20 to about 30% by weight of potassium carbonate,
and (b) an activator or promoter system for the potassium
carbonate, comprising from about 5 to about 15% by weight of
N-secondary butyl glycine as the sole promoter, (c) the balance of
the solution comprising water and additives selected from the group
consisting of antifoaming agents, antioxidants and corrosion
inhibitors, wherein said contacting is conducted at conditions
whereby CO.sub.2 is absorbed in said absorbing solution and the
temperature of the absorbing solution is in the range from about
35.degree. to about 150.degree. C., and the pressure in the
absorber is in the range from about 100 to about 1500 psig; and (2)
regenerating said absorbing solution at conditions whereby CO.sub.2
is desorbed from said absorbing solution, wherein the regeneration
takes place at temperatures ranging from about 35.degree. to about
150.degree. C. and at pressures ranging from about 5 to about 100
psig.
10. The process of claim 9 wherein the absorbing solution from the
regeneration step is recycled for reuse in the absorption step.
11. An aqueous acid gas scrubbing composition comprising: (a) 10 to
about 40% by weight of an alkali metal salt or hydroxide, (b) 2 to
about 20% by weight of the sterically hindered monosubstituted
amino acid, N-secondary butyl glycine and (c) the balance,
water.
12. The composition of claim 11 wherein said alkali metal salt or
hydroxide is potassium carbonate.
13. An aqueous acid gas scrubbing composition comprising: (a) 20 to
30% by weight of potassium carbonate, (b) 5 to about 15% by weight
of N-secondary butyl glycine, and (c) the balance, water.
14. The composition of claims 11, 12 or 13 wherein the composition
additionally includes antifoaming agents, antioxidants and
corrosion inhibitors.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of N-secondary butyl
glycine as a promoter for alkaline salts in "hot pot" type acid gas
scrubbing processes.
2. Description of Related Patents
Recently, it was shown in U.S. Pat. No. 4,112,050 that sterically
hindered amines are superior to diethanolamine (DEA) as promoters
for alkaline salts in the "hot pot" acid gas scrubbing process.
U.S. Pat. No. 4,094,957 describes an improvement to this process
whereby amino acids, especially sterically hindered amino acids,
serve to prevent phase separation of the aqueous solution
containing sterically hindered amines at high temperatures and low
fractional conversions during the acid gas scrubbing process.
One of the preferred sterically hindered amines described in these
patents is N-cyclohexyl 1,3-propanediamine. The bulky cyclohexane
ring on this diamino compound provides steric hindrance to the
carbamate formed at this site thereby favoring the expulsion of
CO.sub.2 during regeneration, thereby leaving the hindered amine
group free to protonate. The primary amino group of this diamino
compound assists in maintaining solubility under lean conditions.
Under lean conditions when there is insufficient carbonic acid
present to protonate the hindered amino group, the molecule would
be insoluble were it not for the primary amino group which forms a
stable polar carbamate ion. However, even the carbamated primary
amino group is insufficient to prevent insolubility of the compound
under very lean conditions and an additional additive, as proposed
in U.S. Pat. No. 4,094,957, an amino acid, is required to maintain
solubility of the diamino compound. This amino acid also
contributes to additional capacity and faster absorption rates for
carbon dioxide, so it therefore acts as a copromoter in addition to
solubilizing the sterically hindered diamino compound. Screening
studies of available amino acids as possible copromoters for
N-cyclohexyl 1,3-propanediamine based on cyclic capacity and rates
of absorption ascertained that pipecolinic acid was one of the best
amino acid copromoters.
Subsequent studies, however, have demonstrated that the
N-cyclohexyl-1,3-propanediamine-pipecolinic acid promoter system
has several shortcomings. Firstly, N-cyclohexyl-1,3-propanediamine
is both chemically unstable and volatile. For example, it degrades
into a cyclic urea in the presence of hydrogen sulfide. In fact,
the rate of cyclic urea formation has been found to be highly
dependent on hydrogen sulfide concentration, a common contaminant
of industrial acid gas streams. The cyclic urea formation from this
diamine is favored by the stability of the six-membered ring
structure of the cyclic urea. In addition to promoter losses due to
cyclic urea formation, which may be a serious problem with hydrogen
sulfide rich streams, the cyclic urea product has limited
solubility, and its separation from solution poses additional
problems. Various techniques for coping with this water insoluble
cyclic urea have been proposed. See, for example, U.S. Pat. Nos.
4,180,548 and 4,183,903. However, these techniques have specific
benefits and problems, e.g., specialized equipment is
necessary.
Pipecolinic acid also has shortcomings, e.g., it is rather
expensive and its picoline precursor is in limited supply.
In view of the commercial potential of using the sterically
hindered amino compounds as described and claimed in U.S. Pat. Nos.
4,094,957 and 4,112,050, there is a need for finding sterically
hindered amino compounds which perform as well as
N-cyclohexyl-1,3-propanediamine but do not have the volatility and
degradation problems of this compound. Also, there is a need for
finding a less costly replacement for pipecolinic acid which
possesses its effectiveness. Preferably, there is a need for
finding a single amino compound which performs as well or nearly as
well as the N-cyclohexyl-1,3-propane-diamine/pipecolinic acid
mixture, but not suffer the preparative cost volatility and
degradation problems of this mixture. Such a discovery would be of
significant technical and economic merit.
Various amino acids have been proposed as promoters for alkaline
salts in the "hot pot" gas scrubbing process. For example, British
Pat. No. 1,305,718 describes the use of beta and gamma amino acids
as promoters for alkaline salts in the "hot pot" acid gas treating
process. These amino acids, however, are not suitable because the
beta-amino acids undergo deamination when heated in aqueous
potassium carbonate solutions. The gamma amino acids form insoluble
lactams under the same conditions. Also, the alpha-amino acid,
N-cyclohexyl glycine, as described in Belgian Pat. No. 767,105,
forms an insoluble diketopiperazine when heated in aqueous
solutions containing potassium carbonate.
SUMMARY OF THE INVENTION
It has now been discovered that N-secondary butyl glycine, a
sterically hindered monosubstituted alpha-amino acid, is an
excellent promoter for alkaline salts in the "hot pot" acid gas
scrubbing process. This amino acid, when used as the sole promoter,
not only provides for high carbon dioxide capacity and high rates
of carbon dioxide absorption, but does not form undesirable
insoluble degradation products as in the case of
N-cyclohexyl-1,3-propanediamine, the beta and gamma amino acids and
the alpha amino acid, N-cyclohexyl glycine. Also, this amino acid
is less volatile than N-cyclohexyl-1,3-propanediamine, thereby the
economy of this promoter is greater than the previously employed
promoters. In addition, this amino acid is superior in terms of
carbon dioxide capacity and rates of absorption for carbon dioxide
than pipecolinic acid and related amino acids. Furthermore,
N-secondary butyl glycine can be prepared from relatively
inexpensive compounds thereby reducing the cost compared to the use
of pipecolinic acid.
Accordingly, in one embodiment of the present invention, there is
provided a process for the removal of CO.sub.2 from a gaseous
stream containing CO.sub.2 which comprises contacting said gaseous
stream (1) in an absorption step with an aqueous absorbing solution
comprising (a) a basic alkali metal salt or hydroxide separated
from the group consisting of alkali metal bicarbonates, carbonates,
hydroxides, borates, phosphates and their mixtures, and (b) an
activator or promoter system for said basic alkali metal salt or
hydroxide comprising at least an effective amount of N-secondary
butyl glycine; and (2) in a desorption and regeneration step,
desorbing at least a portion of the absorbed CO.sub.2 from said
absorbing solution.
As another embodiment of the present invention, there is provided
an acid gas scrubbing composition comprising: (a) 10 to about 40%
by weight of an alkali metal salt or hydroxide, (b) 2 to about 20%
by weight of N-secondary butyl glycine, and (c) the balance,
water.
In general, the aqueous scrubbing solution will comprise an
alkaline material comprising a basic alkali metal salt or alkali
metal hydroxide selected from Group IA of the Periodic Table of
Elements. More preferably, the aqueous scrubbing solution comprises
potassium or sodium borate, carbonate, hydroxide, phosphate or
bicarbonate. Most preferably, the alkaline material is potassium
carbonate.
The alkaline material comprising the basic alkali metal or salt or
alkali metal hydroxide may be present in the scrubbing solution in
the range from about 10% to about 40% by weight, preferably from
20% to about 35% by weight. The actual amount of alkaline material
chosen will be such that the alkaline material and the amino acid
activator or promoter system remain in solution throughout the
entire cycle of absorption of CO.sub.2 from the gas stream and
desorption of CO.sub.2 from the solution in the regeneration step.
Likewise, the amount and mole ratio of the amino acids is
maintained such that they remain in solution as a single phase
throughout the absoption and regeneration steps. Typically, these
criteria are met by including from about 2 to about 20% by weight
of preferably from 5 to 15% by weight, more preferably, 5 to 10% by
weight of this sterically hindered monosubstituted amino acid,
N-secondary butyl glycine.
The aqueous scrubbing solution may include a variety of additives
typically used in acid gas scrubbing processes, e.g., antifoaming
agents, antioxidants, corrosion inhibitors and the like. The amount
of these additives will typically be in the range that they are
effective, i.e., an effective amount.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 graphically illustrates the vapor-liquid equilibrium
isotherms for potassium carbonate solutions activated by equal
nitrogen contents of N-secondary-butyl glycine and diethanola mine
at 250.degree. F. (121.1.degree. C.) wherein the CO.sub.2 partial
pressure is a function of the carbonate conversion.
DESCRIPTION OF THE PREFERRED EMBODIENTS
The term acid gas includes CO.sub.2 alone or in combination with
H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and the oxides
and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons. These
acid gases may be present in trace amounts within a gaseous mixture
or in major proportions.
The contacting of the absorbent mixture and the acid gas may take
place in any suitable contacting tower. In such processes, the
gaseous mixture from which the acid gases are to be removed may be
brought into intimate contact with the absorbing solution using
conventional means, such as a tower packed with, for example,
ceramic rings or with bubble cap plates or sieve plates, or a
bubble reactor.
In a preferred mode of practicing the invention, the absorption
step is conducted by feeding the gaseous mixture into the base of
the tower while fresh absorbing solution is fed into the top. The
gaseous mixture freed largely from acid gases emerges from the top.
Preferably, the temperature of the absorbing solution during the
absorption step is in the range from about 25.degree. to about
200.degree. C., and more preferably from 35.degree. to about
150.degree. C. Pressures may vary widely; acceptable pressures are
between 5 and 2000 psia, preferably 100 to 1500 psia, and most
preferably 200 to 1000 psia in the absorber. In the desorber, the
pressures will range from about 5 to 100 psig. The partial pressure
of the acid gas, e.g., CO.sub.2 in the feed mixture will preferably
be in the range from about 0.1 to about 500 psia, and more
preferably in the range from about 1 to about 400 psia. The
contacting takes place under conditions such that the acid gas,
e.g., CO.sub.2, is absorbed by the solution. Generally, the
countercurrent contacting to remove the acid gas will last for a
period of from 0.1 to 60 minutes, preferably 1 to 5 minutes. During
absorption, the solution is maintained in a single phase.
N-secondary butyl glycine aids in reducing foam in the contacting
vessels.
The aqueous absorption solution comprising the alkaline material,
the activator system comprising the sterically hindered
monosubstituted amino acid, N-secondary butyl glycine, which is
saturated or partially saturated with gases, such as CO.sub.2 and
H.sub.2 S may be regenerated so that it may be recycled back to the
absorber. The regeneration should also take place in a single
liquid phase. Therefore, the presence of the highly water soluble
amino acid provides an advantage in this part of the overall acid
gas scrubbing process. The regeneration or desorption is
accomplished by conventional means, such as pressure reduction,
which causes the acid gases to flash off or by passing the solution
into a tower of similar construction to that used in the absorption
step, at or near the top of the tower, and passing an inert gas
such as air or nitrogen or preferably steam up the tower. The
temperature of the solution during the regeneration step may be the
same as used in the absorption step, i.e., 25.degree.to about
200.degree. C., and preferably 35.degree. to about 150.degree. C.
The absorbing solution, after being cleansed of at least a portion
of the acid bodies, may be recycled back to the absorbing tower.
Makeup absorbent may be added as needed. The use of N-secondary
butyl glycine enables one to maintain a single phase regardless of
the CO.sub.2 content in the acid gas.
As a typical example, during desorption, the acid gas, e.g.,
CO.sub.2 -rich solution from the high pressure absorber is sent
first to a flash chamber where steam and some CO.sub.2 are flashed
from solution at low pressure. The amount of CO.sub.2 flashed off
will, in general, be about 35 to 40% of the net CO.sub.2 recovered
in the flash and stripper. This is increased somewhat, e.g., to 40
to 50%, with the high desorption rate promoter system owing to a
closer approach to equilibrium in the flash. Solution from the
flash drum is then steam stripped in the packed or plate tower,
stripping steam having been generated in the reboiler in the base
of the stripper. Pressure in the flash drum and stripper is usually
16 to about 100 psia, preferably 16 to about 30 psia, and the
temperature is in the range from about 25.degree. to about
200.degree. C., preferably 35.degree. to about 150.degree. C., and
more preferably 100.degree. to about 140.degree. C. Stripper and
flash temperatures will, of course, depend on stripper pressure,
thus at about 16 to 25 psia stripper pressures, the temperature
will preferably be about 100.degree. to about 140.degree. C. during
desorption. Single phase is maintained and facilitated by use of
the N-secondary butyl glycine promoter.
In the most preferred embodiment of the present invention, the acid
gas, e.g., CO.sub.2 is removed from a gaseous stream by means of a
process which comprises, in sequential steps, (1) contacting the
gaseous stream with a solution comprising 10 to about 40 weight
percent, preferably 20 to about 30 weight percent of potassium
carbonate, an activator or promoter system comprising 2 to about 20
weight percent, preferably 5 to about 15 weight percent, more
preferably 5 to about 10 weight percent of the sterically hindered
monosubstituted amino acid, N-secondary butyl glycine, the balance
of said solution being comprised of water, said contacting being
conducted at conditions whereby the acid gas is absorbed in said
solution, and preferably at a temperature ranging from 25.degree.
to about 200.degree. C., more preferably from 35.degree. to about
150.degree. C. and a pressure ranging from 100 to about 1500 psig,
and (2) regenerating said solution at conditions whereby said acid
gas is desorbed from said solution. By practicing the present
invention, one can operate the process above described at
conditions whereby the working capacity, which is the difference in
moles of acid gas absorbed in the solution at the termination of
steps (1) and (2 ) based on the moles of potassium carbonate
originally present, is greater than obtained under the same
operating conditions for removing acid gases from gaseous streams,
wherein said same operating conditions do not include N-secondary
butyl glycine as the promoter. In other words, working capacity is
defined as follows: ##EQU1##
It should be noted that throughout the specification wherein
working capacity is referred to, the term may be defined as the
difference between CO.sub.2 loading in solution at absorption
conditions (step 1) and the CO.sub.2 loading in solution at
regeneration conditions (step 2) each divided by the initial moles
of potassium carbonate. The working capacity is equivalent to the
thermodynamic cyclic capacity, that is the loading is measured at
equilibrium conditions. This working capacity may be obtained from
the vapor-liquid equilibrium isotherm, that is, from the relation
between the CO.sub.2 pressure in the gas and the acid gas, e.g.,
CO.sub.2 loading in the solution at equilibrium at a given
temperature. To calculate thermodynamic cyclic capacity, the
following parameters must usually be specified: (1) acid gas, e.g.,
CO.sub.2 absorption pressure, (2) acid gas, e.g., CO.sub.2
regeneration pressure, (3) temperature of absorption, (4)
temperature of regeneration, (5) solution composition, that is
weight percent N-secondary butyl glycine and the weight percent of
the alkaline salt or hydroxide, for example potassium carbonate,
and (6) gas composition. The skilled artisan may conveniently
demonstrate the improved process which results by use of the
sterically hindered amino acid by a comparison directly with a
process wherein the sterically hindered amino acid is not included
in the aqueous scrubbing solutions. For example, it will be found
when comparing two similar acid gas scrubbing processes (that is
similar gas composition, similar scrubbing solution composition,
similar pressure and temperature conditions) that when the
sterically hindered amines are utilized the difference between the
amount of acid gas, e.g., CO.sub.2 absorbed at the end of step 1
(absorption step) defined above and step 2 (desorption step)
defined above is significantly greater. This significantly
increased working capacity is observed even though the scrubbing
solution that is being compared comprises an equimolar amount of a
prior art amine promoter, such as diethanolamine,
1,6-hexanediamine, etc. It has been found that the use of the
N-secondary butyl glycine of the invention provides a working
capacity which is at least 15% greater than the working capacity of
a scrubbing solution which does not utilize the sterically hindered
amino acid. Working capacity increases of from 20 to 60% may be
obtained by use of the sterically hindered amino acid compared to
diethanolamine.
Besides increasing working capacity and rates of absorption and
desorption, the use of the N-secondary butyl glycine leads to lower
steam consumption during desorption.
Steam requirements are the major part of the energy cost of
operating an acid gas, e.g., CO.sub.2 scrubbing unit. Substantial
reduction in energy, i.e., operating costs will be obtained by the
use of the process utilizing N-secondary butyl glycine. Additional
savings from new plant investment reduction and debottlenecking of
existing plants may also be obtained by the use of N-secondary
butyl glycine. The removal of acid gases such as CO.sub.2 from gas
mixtures is of major industrial importance, particularly the
systems which utilize potassium carbonate activated by the unique
activator or promoter system of the present invention.
While the sterically hindered amines, as shown in U.S. Pat. No.
4,112,050, provide unique benefits in their ability to improve the
working capacity in the acid scrubbing process, their efficiency
decreases in alkaline "hot pot" (hot potassium carbonate) scrubbing
systems at high temperatures and at low concentrations of the acid
gas due to phase separation. Therefore, full advantage of these
highly effective sterically hindered amines cannot always be
utilized at these operating conditions. The addition of an amino
acid, as a cosolvent and copromoter as shown in U.S. Pat. No.
4,094,957, solves the problem of phase separation and enables a
more complete utilization of sterically hindered amines as the
alkaline materials activator or promoter. Many of the disclosed
amino acids when used alone, such as pipecolinic acid, while
soluble in these alkaline systems, are not as effective as
activators in acid gas scrubbing processes as the other sterically
hindered amines. Subsequent tests have confirmed that most amino
acids are not as effective as N-cyclohexyl 1,3-propanediamine.
Therefore, it was not expected that N-secondary butyl glycine, as
the sole promoter, would provide high working capacity and high
rates of CO.sub.2 absorption.
The absorbing solution of the present invention, as described
above, will be comprised of a major proportion of two alkaline
materials, e.g., alkali metal salts or hydroxides and a minor
proportion of the amino acid activator system. The remainder of the
solution will be comprised of water and/or other commonly used
additives, such as anti-foaming agents, antioxidants, corrosion
inhibitors, etc. Examples of such additives include arsenious
anhydride, selenious and tellurous acid, protides, vanadium oxides,
e.g., V.sub.2 O.sub.3, chromates, e.g., K.sub.2 Cr.sub.2 O.sub.7,
etc.
The N-secondary butyl glycine compound useful in the practice of
the present invention is either available commercially or may be
prepared by various known procedures. N-secondary butyl glycine has
the CAS Registry Number of 58695-42-4 and is mentioned as an
intermediate in several U.S. patents, e.g., U.S. Pat. Nos.
3,894,036; 3,933,843; 3,939,174 and 4,002,636, as well as the
published literature (Kirino et al., Agric. Biol. Chem., 44(1), 31
(1980), but nothing is said in these disclosures about the
synthesis of this amino acid or its use as a carbonate promoter in
acid gas scrubbing processes.
A preferred method for preparing N-secondary butyl glycine
comprises reacting glycine under reductive conditions with methyl
ethyl ketone in the presence of a hydrogenation catalyst. This
reaction produces the sterically hindered monosubstituted amino
acid N-secondary butyl glycine in nearly quantitative yields. This
process is more fully described and claimed in U.S. Ser. No.
321,058, filed concurrently herewith, entitled, "Amino Acids and
Process for Preparing the Same" (G. Sartori and W. Thaler), the
disclosure of which is incorporated herein by reference.
The invention is illustrated further by the following examples
which, however, are not to be taken as limiting in any respect. All
parts and percentages, unless expressly stated to be otherwise, are
by weight.
EXAMPLE 1
"Hot Pot" Acid Gas Treating Process
The reaction apparatus consists of an absorber and a desorber as
shown in FIG. 1 of U.S. Pat. No. 4,112,050, incorporated herein by
reference. The absorber is a vessel having capacity of 2.5 liters
and a diameter of 10 cm, equipped with a heating jacket and a
stirrer. A pump removes liquid from the bottom of the reactor and
feeds it back to above the liquid level through a stainless-steel
sparger. Nitrogen and CO.sub.2 can be fed to the bottom of the cell
through a sparger.
The desorber is a 1-liter reactor, equipped with teflon blade
stirrer, gas sparger, reflux condenser and thermometer.
The following reagents are charged into a 2-liter Erlenmeyer
flask:
92 g of N-secondary butyl glycine
225 g of K.sub.2 CO.sub.3
433 g of water
When all solid has dissolved, the mixture is put into the absorber
and brought to 80.degree. C. The apparatus is closed and evacuated
until the liquid begins to boil. At this point, CO.sub.2 is
admitted into the absorber. Thirty-two (32) liters of CO.sub.2 is
absorbed.
The rich solution so obtained is transferred to the desorber and
boiled for one hour, during which time 28 liters of CO.sub.2 is
desorbed. The regenerated solution so obtained is put into the
absorber and cooled to 80.degree. C. The apparatus is closed and
evacuated until the liquid begins to boil. At this point CO.sub.2
is admitted. 29.6 liters of CO.sub.2 is absorbed, of which 13
liters is absorbed in the first minute.
EXAMPLE 2
The procedure of Example 1 is repeated after replacing N-secondary
butyl glycine with an equimolar amount of other amino compounds,
including structurally related sterically hindered amines and amino
acids, and correcting the amount of water in order to have a total
initial weight of 750 g. The results of these tests are shown in
Table I.
TABLE I ______________________________________ CO.sub.2 SCRUBBING
BY AMINO ACID PROMOTED K.sub.2 CO.sub.3 SOLUTIONS Liters of
CO.sub.2 Absorbed Into Regenerated Solution Amino acid Total First
Minute ______________________________________ N--sec. butyl glycine
29.6 13 N--cyclohexyl glycine 30.8 14 N--isopropyl glycine 27.5 10
N--(2-amyl)-glycine 26.5 17 N--sec. butyl alanine 15 3 N--isopropyl
alanine 25.2 4 Pipecolinic Acid 22.5 8
______________________________________
The data in Table I show that N-secondary butyl glycine and
N-cyclohexyl glycine are superior promoters to the other tested
sterically hindered amines and amino acids of similar
structure.
In view of the data shown in Table I, additional tests were
conducted with N-secondary butyl glycine and N-cyclohexyl glycine
to ascertain their suitability in large scale acid gas scrubbing
operations.
EXAMPLE 3
(a) Aging Studies in CO.sub.2 Scrubbing Apparatus
The following experiments are carried out to ascertain the
stability of N-secondary butyl glycine and N-cyclohexyl glycine
under accelerated-simulated acid gas treating conditions.
The following reagents are charged into a stainless-steel bomb:
121 g of N-secondary butyl glycine
433 g of KHCO.sub.3
540 g of H.sub.2 O
The bomb is put into an oven and heated at 120.degree. C. for 1000
hours. Then the content is discharged into a 2 liter flask and
refluxed for several hours.
750 g is taken and subjected to an
absorption-desorption-reabsorption cycle as described in Example 1.
27.9 liters of CO.sub.2 is absorbed into the regenerated solution,
10 liters being absorbed in the first minute.
Comparison of this result with that obtained with the fresh
solution, described in Example 1, shows that the aging process does
not lead to a significant loss of activity.
If the aging experiment is carried out after replacing N-secondary
butyl glycine with the equivalent amount of N-cyclohexyl glycine,
145 g, and reducing the water to 516 g in order to have the same
total weight, a considerable amount of solid, identified as
1,4-bis-cyclohexyl-2,5-diketopiperazine is formed. An attempt to
carry out an absorption-desorption cycle causes plugging of the
unit.
(b) Aging Under CO.sub.2 and H.sub.2 S
The following reagents are charged into a stainless-steel bomb:
121 g of N-secondary butyl glycine
24 g of K.sub.2 S
390 g of KHCO.sub.3
544 g of water
The bomb is put into an oven and heated at 120.degree. C. for 1000
hours. Then the content is discharged into a 2 liter flask and
refluxed for several hours.
765 g is taken and subjected to an
absorption-desorption-reabsorption cycle as described in Example 1.
28.9 liters of CO.sub.2 is absorbed into the regenerated solution,
10 g being absorbed in the first minute. Comparison of this result
with that obtained with the fresh solution, described in Example 1,
shows that the aging process leads to only a slight loss of
activity.
The excellent stability under the aging conditions shown above for
the N-secondary butyl glycine coupled with its good performance as
a promoter, demonstrates the desirability of using this amino acid
rather than N-cyclohexylglycine.
EXAMPLE 4
Vapor-liquid equilibrium measurements were carried out to confirm
that N-secondary butyl glycine leads to a broadening of cyclic
capacity (as defined in U.S. Pat. No. 4,112,050, incorporated
herein by reference) as compared to diethanolamine owing to a shift
in the equilibrium position.
The vapor-liquid equilibrium measurements are made by first
preparing the following solution:
______________________________________ 73.6 g of N--secondary butyl
glycine 150.0 g of K.sub.2 CO.sub.3 376.4 g H.sub.2 O 600.0 g
______________________________________
The solution is charged into a one-liter autoclave, equipped with
stirrer, condenser, inlet and outlet tube for gases and
liquid-sampling line. The autoclave is brought to 250.degree. F.
while blowing through the solution a mixture of 20 mol % CO.sub.2
and 80 mol % He. The rate at which the gaseous mixture is fed is
0.2 liter/min.
The pressure is 300 psig. When the outgoing gas has the same
composition as the entering gas, equilibrium has been reached. A
sample of liquid is taken and analyzed. The CO.sub.2 content is
13.0 wt. %, the K content is 13.6%, from which a carbonation ratio
of 1.70 is calculated. By carbonation ratio, it is meant the molar
ratio of CO.sub.2 absorbed to initial K.sub.2 CO.sub.3.
The operation is repeated several times, changing the composition
of the gas and the total pressure. If the partial pressures of
CO.sub.2 are plotted against the carbonation ratios, the curve
shown in FIG. 1 is obtained.
Using the same procedure, the vapor-liquid equilibrium curve is
determined, using diethanolamine in an amount equivalent to the
total amino acid amount used above. The resulting vapor-liquid
equilibrium curve is also shown in FIG. 1.
In the interval of P.sub.CO.sbsb.2 studied, i.e., from 0.08 to 300
psia, N-secondary butyl glycine leads to a larger cyclic capacity
than diethanolamine.
While the invention has been described in connection with specific
embodiments thereof, it will be understood that it is capable of
further modification, and this application is intended to cover any
variations, uses, or adaptations of the invention following, in
general, the principles of the invention and including such
departures from the present disclosure as come within known or
customary practice in the art to which the invention pertains and
as may be applied to the essential features hereinbefore set forth,
and as fall within the scope of the invention.
* * * * *